The present invention relates to a novel process for the preparation of substituted pyranes of the general formula I 
wherein R1, R2 and R3 are lower alkyl.
The compounds of formula I belong to a known class of compounds. For instance, the compound of formula I wherein each of R1, R2 and R3 is methyl, 2-ethinyl-tetrahydro-2,6,6-trimethylpyran, is a known intermediate, e.g., in the manufacture of flavours and has been prepared so far from dehydrolinalool by ring closure catalyzed by tungsten, molybdenum or polyphosphoric acid (Strickler et al., Helv. Chem. Acta 1966, 49, 2055; Erman et al., Tetrahedron 1976, 34, 2981 and Belgian Patent No. 852 918).
In accordance with the present invention it has been found that the compounds of formula I can be conveniently prepared by exposing a compound of the general formula II 
wherein R1, R2 and R3 are as above,
in the presence of a solid acid to elevated temperature or irradiation with microwaves.
The process of the present invention proceeds in high yields and in short reaction time. Further, the process of the present invention has the advantage that the use of a solvent and unwanted pressure increase in closed reaction vessels is avoided.
The term xe2x80x9cmicrowavexe2x80x9d as used herein refers to the region of the electromagnetic spectrum having frequencies of 30 GHz to 300 MHz thus corresponding to wavelengths of 1 cm to 1 m. In order not to interfere with wavelengths for Radar (1 cm-25 cm), household or industrial microwave heaters are required to operate at either 12.2 cm (2.45 GHz) or 33.3 cm (918 MHz). Thus, in a preferred embodiment of the invention, the term microwaves refers particularly to such wavelenghts. In the process of this invention, conventional microwave equipment can be used. Microwave equipment suitable in the process of this invention is supplied, e.g., by the firms MLS, Leutkirch, Germany (Lavis Multiquant 1000); or MILESTONE Inc., Monroe, Conn. 06468, USA (Ethos reactors). Conveniently, the irradiation in the process of this invention is carried out applying a power of irradiation of from about 600 W to 1200 W, more preferably from about 800 W to about 1000 W.
As used herein, xe2x80x9csolid acidxe2x80x9d means an ordered solid having defined acidic centers selected from Lewis and Bronstedt centers Such centers may be engineered to control which reaction product is favored in a particular reaction. Thus, solid acids, which may be used as catalysts in the present invention not only accelerate the rate of the reaction, but in reaction systems where multiple products may be formed, they may strongly influence which of the products is produced in the greatest amount. The solid acid used as a catalyst in the present invention is suitably a strong organic acid, such as a polymeric sulfonic acid, e.g., a polyperfluoroalkylene sulfonic acid, particularly Deloxan(copyright) ASP (Degussa, Frankfurt/M., Germany) or Nafion(copyright) NR 50 (DuPont, Wilmington, Del.,USA), or an anionic ion exchange resin such as Amberlyst(copyright) 15 (Rohm and Haas, Philadelphia, Pa.,USA); or an inorganic acid, such as sulfuric acid/silizium dioxide, or silicates such as zeolithes, e.g., Zeocat(copyright), and Wessalith(copyright) types (Degussa), montmorillonites, e.g. Montmorillonit K 10 and KSF (Fluka, Buchs, Switzerland) and mesoporous (pore size 2-50 nm) metal-doped silica gels (Degussa).
Particularly preferred are microporous (pore size less than 2 nm) zeolithes such as Wessalith(copyright)Day P and and Zeocat(copyright) types such as Zeocat Z6-05-02.
In a preferred aspect, R1, R2 and R3 are each methyl. However, any compound of the general formula I wherein R1, R2 and R3 are, independently, lower alkyl, can be prepared by the process of this invention. The term lower alkyl as used herein denotes straight or branched chain alkyl groups having up to 7 carbon atoms, such as methyl, ethyl, propyl, isopropyl, n-butyl, sek.butyl, tert.butyl, n-pentyl and isomers thereof.
If the process of the presence invention is carried out by exposing the compound of formula II to elevated temperature, the compound of formula II is heated, in the presence of a solid acid, to a temperature of about 40xc2x0 C. to about 100xc2x0 C., preferably of about 60xc2x0 C. to about 100xc2x0 C. However, in a preferred aspect, the present invention relates to the preparation of a compound of formula I wherein a compound of the general formula I is excposed to microwave irradiation.
The process according to the present invention is carried out in dry state, i.e., in the absence of solvents. Prior to exposure to elevated temperature or irradiation, the starting compound of formula II is suitably thoroughly mixed with the catalyst, e.g., by suspending the catalyst in a solution of the compound of formula II, e.g., in dichloromethane or an ether such as methyl-tert.butyl ether, with stirring and subsequent removal of the solvent or, if the catalyst is a porous material, simply adding the compound of formula II to the catalyst. The catalyst is suitably used in an amount of about 0.5 to about 10, preferably about 1 to about 2 parts of weight per part of weight of compound of formula II.